Many electronic devices such as spectrum analyzers use swept frequency oscillators as local oscillators in order to repeatedly tune over a particular band of frequencies. One of the drawbacks of analog sweep oscillators is that they drift in frequency, and this lack of stability in the local oscillator is reflected in the lack of stability in the frequency readout of the spectrum analyzer. This problem has been solved in the past by using a local oscillator which is synthesized or digitally swept in discreet steps. This, however, has the disadvantage that it is much more expensive than an analog swept oscillator and the sweep has a number of steps in it rather than being one, smooth, continuous curve.
Other prior art spectrum analyzers have attempted to overcome the problem of instability in the first local oscillator by phase locking the first local oscillator to a fixed reference and then sweeping one of the lower frequency local oscillators over a narrower band. Since the lower frequency local oscillator has greater stability than the broad frequency range first local oscillator frequency instabilities are reduced. An example of this technique is shown in U.S. Pat. No. 3,482,181.
Another technique for stabilizing a swept frequency oscillator is shown in U.S. Pat. No. 3,144,623 in which the sweeping oscillator is injection locked at the start of the sweep and nulls are counted during the sweep for correction of the oscillator frequency. One of the disadvantages of this technique is that because no error voltage is stored there is a discontinuity at the start of the sweep until the loop corrects the oscillator. This closed loop sweeping technique uses a considerable amount of circuitry for linearity correction which, with some oscillators and for some applications, is unnecessary and thus unjustifiably expensive.